We plan to investigate the role of MHC-gene products on the function of T cells involved in T-B collaboration in vivo. We have three aims: Firstly it is hoped to prove that induction of T helper cells in vivo depends on antigen presentation by MHC-compatible macrophages. Secondly, we will test the notion that the unrestricted helper function of T cells raised in a chimeric environment is due to the generation of an aberrant population of T helper cells which can recognize antigen presented in the context of foreign MHC determinants. Thirdly, with the aid of chimeric mice attempts will be made to raise antisera hypothetical recognition sites which determine specificty for MHC associated antigen.